1. Field of the Invention
The present invention relates to electrostatographic copiers and printers, and more particularly, to devices for removing or scavenging unwanted carrier particles from toner-developed images on the image-bearing member of such copiers and printers.
2. Background of the Invention
The process of producing or reproducing copies of images in an electrostatographic copier or printer involves moving an imaging member, in the form of a rigid drum or flexible web, past a series of process stations. As this occurs, the imaging member is first charged, and then imagewise exposed to form a latent electrostatic charge image thereon. The latent image is thereafter developed or made visible by moving it past a development station or apparatus where charged, pigmented toner particles, from development material which is held in a sump portion at the development station, are attracted to the latent image charges. The developed or toner image is subsequently transferred, at a transfer station, to a suitable receiver, such as a copy sheet of paper which is thereafter advanced through a fusing station. At the fusing station, the toner particles on the copy sheet, particularly those forming the desired image, are heated and fused. Meantime, any particles remaining on the imaging member are thereafter removed, at a cleaning station using a cleaning apparatus such as a fiber brush, prior to again reusing the imaging member as above to form and transfer images.
The development material being held in the sump portion at the development station may be single component, in that it consists entirely of toner particles. It also may be multiple component, for example two-component, which consists of an admixture of toner particles and carrier particles. In the latter case, the carrier particles are stirred together with the toner particles in order to triboelectrically charge both types of particles. The charged developer material, consisting of such charged particles, is then moved within the development station or apparatus so as to bring the toner particles into transfer proximity with the latent image on the image-bearing member during the development step of the electrostatographic process as described above.
During this development step, however, some of the carrier particles in the development material unfortunately are undesirably also transferred to the latent image on the image-bearing member. Such undesirably transferred carrier particles, referred to as DPU (developer pickup) particles, if not removed, will cause image defects such as black spots, image voids and halftones, when such particles (DPU) are transferred to the receiver for fusing.
In attempts to prevent such image defects, various conventional devices have been developed for removing such undesirable or unwanted carrier particles (DPU) from the image-bearing member prior to the toner image thereon being transferred to the receiver or copy sheet. For example, conventional roller and fixed magnet-type scavenging devices are disclosed in commonly assigned U.S. Pat. Nos. 3,457,900 issued July 29, 1969, and 3,543,720 issued Dec. 1, 1970, both in the names of R. A. Drexler et al. Each of these conventional roller-type scavenging devices includes a strong magnet positioned within a rotatable roller that is supported, and driven, supposedly spaced a small desired distance from a developed image on the image-bearing surface of the image-bearing member. As such, each such scavenging device includes rotatably supporting means, as well as driving means, thereby making it relatively more expensive, and more susceptible to moving component failure.
In addition, it has been found that such a roller-type scavenging device tends to sag about its middle when supported (at its ends) for such desired spacing across the width of the image-bearing surface. Such sagging detrimentally increases the spacing of the roller from the image-bearing surface towards the middle of the roller, and hence decreases its scavenging effectiveness thereat.
The scavenging effectiveness of such a roller-type scavenging device is also hampered because the roller can only scavenge or pick up DPU particles from a relatively narrow image area or band that corresponds to a footprint the roller would make on such image area when the image-bearing surface is tangential to the roller. Additionally, such a roller-type scavenging device undesirably accumulates some of the DPU particles scavenged from the image-bearing surface. Such accumulation occurs because of a tendency of some DPU particles to continue rotating around and around on, and with, the surface of the scavenging roller, rather than to desirably fall off. Such accumulation, for example, detrimentally effects the continued ability of the scavenging device to attract new DPU particles from the image-bearing surface, thereby resulting in poor quality copies.